Page 161 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

acts closer to a short circuit, which means that high-frequency signals appear at Rl.

The signal at Ri is amplified by a gain of 1 and fed back to the input via R2.
So let's take a look at what happens if the signal at the input is a DC voltage. Cl
will block the DC voltage, and the voltage at Ri will be zero. The output of the
amplifier then will be zero as well, which grounds one end of R2. This means that
at DC the input of the gyrator is just a resistor, R2. Note that a coil at DC is also
just the internal resistance, which means that a coil can be modeled at DC as a

resistor as well.
For a high-frequency signal at the input of the gyrator, Cl passes most or all of the
input signal into Rl. One can say that the high-frequency signal at the input and
output of the amplifier is really close to or the same as the signal appearing at the
input of the gyrator circuit. This then means that signal voltage on R2 that is

connected to the amplifier is really about the same signal voltage at the input of
the gyrator. But R2 is also connected to the input. If we assume that R1 is very
large and does not load the input signal, then most or all of the signal current
flowing to the gyrator is through R2. But, if the signal voltage on both ends of R2 is
almost the same or is the same, then there is no net current flowing into R2. Recall
Ohm's law, which says that it is the potential difference (voltage difference) across
a resistor that determines the current flowing into the resistor. If the voltage is the

same on both ends of R2, then the potential difference is zero, and thus there is no
current through R2. So, at high frequencies, there is little or no current flowing into
R2 or the gyrator. At high frequencies the gyrator circuit looks like an open circuit
or a very high impedance circuit.

But isn't this the same as the effect on a coil at high frequencies? A high-frequency
signal into a coill will measure as a very high-impedance device or as an open
circuit. Thus the gyrator circuit is "equivalent" to a coil or inductor.
The gyrator circuit in Figure 11-4 has an inductance of L = Cl x Rl x R2. For
example, if Cl = 1,000 pF, R1 = 1,000

, and R2 = 50

(R2 is the equivalent internal resistance of a coil), then this gyrator has an
inductance of 50 JJH. However, in practice, the Q of this gyrator circuit usually does
not exceed 10. Note that the Q values of antenna coils and IF transformers are at
least 50 typically. So the gyrator circuit in Figure 11-4 is not quite suitable for an IF
filter, for example.

But there is another type of gyrator circuit that has two amplifiers and works as a
generalized impedance converter (GIC). Gyrators using a GIC topology generally
have a Q of 50 or more. (Figure 11-6B shows a gyrator circuit using op amps U7 A
and U7B.)